The present invention relates generally to apparatus for accurately measuring a voltage of a rechargeable battery in systems where there is no direct access to battery terminals.
Many types of electronic devices such as computers and data storage devices rely on batteries for backup during power interruptions. Other, portable electronic devices such as laptop computers rely on batteries for main power when the portable device is not plugged into an AC power source. Typically, the battery is rechargeable. It is important to accurately charge the battery to its optimum voltage level and this requires an accurate, real time measurement of the battery voltage during charging.
In the prior art there have been difficulties in accurately measuring the battery voltage during charging, in systems where the battery terminals cannot be accessed directly. For example, the battery or battery pack may be plugged into a printed circuit board to form a battery module, so there is a series resistance in lead wires between the battery terminals and the plug and at the plug itself. Also, the printed circuit board may be inserted into a rack using an edge connector, so there is a series resistance in wire traces on the printed circuit board between the plug and the edge connector and at the edge connector itself. The voltage sensor may be located on another printed circuit board. This other printed circuit board may be connected to the rack using an edge connector, so there is a series resistance in the wire traces between the voltage sensor and the edge connector and at the edge connector itself. Likewise, the charger may be located on a third printed circuit board with an edge connector, or on the same printed circuit board as the voltage sensor, so there is a series resistance in the wire traces between the charger and the edge connector and at the edge connector itself. There are also wire traces on the rack between the different edge connectors of the different printed circuit boards, so there is a series resistance caused by these wire traces. Some of the series resistances in the system cause significant voltage drops during charging, if substantially current passes through them. Heretofore, these voltage drops have prevented an accurate, remote measurement of the battery voltage by the voltage sensor, during charging.
An apparatus, generally designated 8 according to the Prior Art, for measuring battery voltage during charging is schematically illustrated in FIGS. 1-4. A battery pack 10 has three lead wires, one for fast charging from fast charge circuitry 12 . By way of example, the fast charge circuit 12 supplies 0.8 DC amps of charging current, although the amount depends on the type of battery to be charged and other factors. Another lead wire of the battery pack is for trickle charge from a pulsed trickle charge circuit 14, a voltage sense input to a charge control circuit 16, a voltage sense input to a “gas gauge” circuit 18 and to supply a load 20. By way of example, the trickle charge circuit 14 supplies 0.2 amps of pulsed current, although the magnitude of the current and the duty cycle also depend on the type of battery and other known factors. The “gas gauge” measures the battery voltage in conjunction with the current that flows into the battery and flows out of the battery, for purposes of estimating the remaining amount of time the device can be run on the battery. In the illustrated example, the battery 10 drives a DC to DC converter which supplies a load, for example, a data storage device, although the type of load is variable. The third wire lead of the battery pack is the current return for the battery pack via a discreet series resistor 22. The series resistor 22 is small, for example, 0.2 ohms, and can be used to monitor the battery current.
The battery pack 10, with its three leads, is plugged into a printed circuit board 31 and encased to form a battery module 36 (FIG. 2). By way of example, the battery pack plug is a three-pronged type, and the printed circuit board 31 has an edge connector 30a,b,c for connection to a rack. The wire leads between the battery terminals and the plug, wire traces on the printed circuit board between the plug and the edge connector, and the edge connector itself cause a series resistance associated with the battery module. The edge connector of the printed circuit board 31 is connected to a mating connector 30a′, b′, c′ of a rack 33 (FIG. 4). A printed circuit board 32 (FIG. 3) contains fast charge circuit 12, pulse trickle charge circuit 14, charge control circuit 16, gas gauge 18, DC to DC converter circuit 20, a battery current sense resistor 22 and a ground path. Printed circuit board 32 has a connector 40a′,b′,c′ and is connected into rack 33 via a mating connector 40a,b,c. (Alternately, the fast charge circuit 12, trickle charge circuit 14, charge control circuit 16, gas gauge 18 and load 20 could be contained on different printed circuit boards which insert into respective connectors of the rack 33 via respective edge connectors or other types of connectors. Different connectors are also shown in FIG. 4 for this purpose.) Wire traces on the rack between the different connectors cause a series resistance 34a,b,c schematically shown in FIGS. 1 and 4.
During fast charging, considerable current is supplied to the battery 10 from fast charge circuit 12. This considerable current passes through connector 40a,a′, resistor 34a and connector 30a,a′ through the battery, through connector 30c,c′, resistor 34c and connector 40c,c′ and through battery current sense resistor 22 to ground. Charge control circuit 16 measures the voltage from connector 40b′ to ground. During fast charging, the DC-DC converter is disabled (at ENable input) and/or a diode 23 at the output of the DC-DC converter is reversed biased by a DC voltage, Vcc, supplied by the AC source via an AC-DC converter (not shown). Therefore, during fast charging, the DC-DC converter does not draw any current. Also, the impedance at the input of charge control circuit 16 is always very high. So, during fast charging, very little current flows through connector 30b,b′ resistor 34b and connector 40b,b′. Consequently, there is negligible voltage drop across connector 30b,b′ resistor 34b and connector 40b,b′ during fast charging. However, the voltage drop across connector 30c,c′ resistor 34c and connector 40c,c′ and current sense resistor 22 increases the voltage input to the charge control circuit 14, and this distorts the measurement of the battery voltage.
During battery discharge, there is considerable current flowing from ground, through resistor 22 and into the battery, and from the battery through connector 30b,b′ resistor 34b and connector 40b,b′ into the DC-DC converter. This causes a significant voltage drop across connector 30b,b′ resistor 34b and connector 40b,b′ and decreases the voltage input to the gas gauge 18. (The gas gauge does not consider the voltage drop across resistor 22 because of the location of the reference for the gas gauge, and also because the voltage drop across resistor 22 is internally compensated by the gas gauge.) This also distorts the measurement of the battery voltage.
Accordingly, an object of the present invention is to provide an apparatus for accurately measuring a voltage of a battery during charging, in a system where the battery terminals are not directly accessible.
Another object of the present invention is to provide an apparatus of the foregoing type where the battery is plugged into a module, and the module is inserted into a rack.
Another object of the present invention is to provide an apparatus of the foregoing type which is adapted to a three lead battery module.